1. Field of the Invention
This invention relates to an improved Synthetic Terrain Generator (STG), and more particularly to an STG which has the capability of continually generating a clear image of a simulated terrain as the eyepoint viewing the terrain varies over a wide range of altitudes.
2. Description of the Prior Art
Use of an aircraft simulator is a modern well known method of training aircraft personnel. In a simulator the trainee is placed in a realistically recreated aircraft environment such that the trainee feels he is in and operating an actual aircraft without leaving the ground.
An important element that is used to create this realism is the visual display system. Many training judgments depend on the student's response to a visual cue. During flight operation a pilot views his external environment through his aircraft window. For instance, while taking off and landing he scans the terrain and operates his controls to properly maneuver his aircraft.
Some modern simulators employ synthetic terrain generators (STG) of the type described in U.S. Pat. No. 3,911,597 (hereinafter referred to as Millard). The STG utilizes the viewing surface of a raster scanned television display to simulate a window view. A line of sight from an eyepoint through the screen of the display to the synthetic terrain is electronically represented. When this line of sight intersects a grid line of the synthetic terrain an electronically defined null relationship is established. A video pulse representing the intersection of the line of sight with the grid line is provided to display means for presentation of the grid intersection point on the viewing surface. The resultant image on the display is a grid pattern composed of two intersecting groups of grid lines defining a synthetic terrain.
The null relationship is essentially a comparison of signals representing terms defining the line of sight and signals representing the grid line locations. One factor in the line of sight terms is the altitude (H) of the eyepoint above the ground reference plane upon which the grid pattern is located. The altitude plays a key role. The grid pattern must become smaller as the eyepoint gets higher in altitude and further away from the ground surface being viewed. In the same manner, the grid pattern must enlarge as the altitude decreases. For altitudes employed by modern aircraft simulators a clear representation on the viewing surface of the grid pattern must be provided so that an accurate image can be viewed by a trainee. Poor images defeat the purpose of simulating an aircraft's environment.
In the prior art STG a north-south video generator and an east-west video generator, one generator for each horizontal direction defining the grid pattern, are each responsive to a signal representative of the altitude of the eyepoint. Each is also responsive to signals representative of vertical earth direction cosine values, its respective horizontal earth direction cosine signal, and signals representative of the velocity of the aircraft. A signal is then generated within each video generator defining the line of sight in terms of those signals feeding the video generator. In generating the line of sight, the magnitude of the signal (h) that represents the altitude (H) increases and decreases linearly in direct relation to an increase or decrease in altitude. Because the altitude signal (h) varies linearly with altitude a disadvantage of the prior art STG becomes apparent.
Over broad altitude ranges a maximum (h) signal corresponding to a high altitude value would drop to a very small magnitude (h) signal at some low altitude value. For example, if 10 volts corresponded to an altitude of 10,000 meters, 10 milli-volts would correspond to an altitude of 10 meters. On the other hand, if a maximum signal corresponded to some low altitude the signal level would drop linearly to a very small magnitude at the high altitude.
When very small signals are processed signal strength levels become comparable to noise levels. Subsequent signal processing cannot distinguish signal from noise. In the prior art STG, weak altitude signals result in noisy display signals causing a blurred grid pattern on the viewing surface of the raster scanned television display at those values of altitude where the altitude signal is weak.
With aircraft and other type simulators being extensively used as training devices, a need arises for clearly defined visual images. It therefore becomes particularly necessary to provide an STG which has the capability of continually generating a clear image of a simulated terrain as the eyepoint viewing the terrain varies over a wide range of altitudes.